SUMMARY Installing the KDE snap, logging out of the Ubuntu session, then logging into the Plasma snap session, then logging out of that, then back into the Ubuntu session, removes window controls from Deb-based apps. STEPS TO REPRODUCE 1. Run `snap install --edge --devmode plasma-desktop` on a snap-enabled system. 2. Run `sudo wget https://metadata.neon.kde.org/snap/plasma-snap.desktop -O /usr/share/xsessions/plasma-snap.desktop` 3. Log out of your current session and log into Plasma (snap) 4. Log out of the Plasma session and log into the Ubuntu session 5. Open Files, Terminal, Text Editor, other Deb-based apps OBSERVED RESULT The window controls have gone, except, apparently, for Firefox. EXPECTED RESULT The window controls are there as previous. SOFTWARE/OS VERSIONS Linux/KDE Plasma: Ubuntu 18.10, core 2.36.3+git1063.bbf4a49~ubuntu16.04.1 (6148) [edge] KDE Plasma Version: master+3ffadd6 (4) [edge] KDE Frameworks Version: 5.47.0 (27) [stable] ADDITIONAL INFORMATION Via Tweaks I had customized my window controls in the Ubuntu session from the default, they were on the left and in the order (left-to-right): close, minimize, maximize.
On reflection, this may be because I logged into the Ubuntu on Wayland session rather than the Ubuntu snap, which caused an apparent change (it also meant that most Flatpaks are using Adwaita rather than Yaru...)
The no window controls issue was resolved by a reboot but it's not ideal that a reboot is effectively required when this isn't made clear or prompted (the Adwaita Flatpak issue that I mentioned in Comment 1 may still just be Wayland).
What do you mean by "window controls"?
The close, minimize, maximize buttons - X, _, square.
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At this this time the plasma-desktop snap is not maintained nor supported. I do not have an ETA for when a new one will be released.